1. Field of the Invention
The present invention relates to a method of arranging an optical pickup system and an optical recording and/or reproducing apparatus having the arranged optical pickup system.
2. Description of the Related Art
FIG. 1 shows an example of the optical configuration of an optical pickup. Referring to FIG. 1, the optical pickup includes a light source 3 which emits light of a predetermined wavelength, a plate-type beam splitter 5 which changes a propagation path of a light beam emitted by the light source 3, an objective lens 7 which focuses an incident light beam to a spot on a recording surface of an optical disc 1, and a photodetector 9 which detects an information signal and an error signal by receiving the light beam that is reflected from the recording surface of the optical disc 1 and has sequentially passed through the objective lens 7 and the plate-type beam splitter 5. The optical pick-up, as shown in FIG. 4, is arranged such that all components of the optical pickup system, except for the objective lens 7, are approximately parallel to the optical disc 1 by redirecting the optical path by 90 degrees with a reflecting mirror 2. In FIG. 1, reference numerals 4, 6, and 8 refer to a grating, a collimating lens, and a concave lens.
The grating 4 is disposed between the light source 3 and the plate-type beam splitter 5 to diffract the light beam emitted from the light source 3 into 0th-order and ±1st-order beams, so as to detect a tracking error signal by a three-beam method.
FIG. 2 illustrates that in a normal state, a main light beam corresponding to the 0th-order diffracted light beam passing through the grating 4 is focused on a main track 1a as a main light beam spot S0, while sub-beams corresponding to the ±1st-order beams diffracted by the grating 4 are centered as sub-beam spots S1 and S2 approximately symmetrical to radial and tangential directions relative to the main light beam spot S0, on positions slanted with respect to the main track 1a. The main light beam is reflected/diffracted from the optical disc 1 to produce a reflected/diffracted light beam which will be described later. In FIG. 2, a dotted line is used to distinguish one track from another.
Where the grating 4 is provided to detect a tracking error signal by the three-beam method, the photodetector 9 may comprise a main photodetector and a pair of sub-photodetectors to detect a main light beam and sub-beams reflected from the optical disc 1, respectively. The collimating lens 6 is disposed between the plate-type beam splitter 5 and the objective lens 7 so as to convert a divergent light beam emitted from the light source 3 into a parallel light beam. The concave lens 8 is positioned between the plate-type beam splitter 5 and the photodetector 9 to spread out a light beam focused on the photodetector 9. The concave lens 8 is inclined in a direction opposite to the direction in which the plate-beam splitter 5 is tilted so as to remove coma aberrations on a light beam passing through the plate-type beam splitter 5.
In the optical pickup described above, the light beam emitted from the light source 3 is reflected by the plate-type beam splitter 5 and proceeds toward the optical disc 1, and the light beam reflected from the optical disc 1 transmits through the plate-type beam splitter 5 and travels to the photodetector 9. Hereinafter, an axis of propagation path of the light beam transmitted through the plate-type beam splitter 5 to the photodetector 9 is called an optical path axis c.
FIG. 3 illustrates that since the plate-type beam splitter 5 is tilted with respect to the optical path axis c, an optical length in a horizontal direction h of the light beam transmitted through the plate-type beam splitter 5 remains constant regardless of an incident location of the light beam on the beam splitter 5, while an optical length in a vertical direction v varies depending on the location. This causes astigmatism due to difference in focal points in the horizontal and vertical directions h and v. Here, an axis in the vertical direction v is one axis, i.e., a major axis of an oval light beam suffering from astigmatism, and an axis parallel to the vertical axis is an astigmatism axis.
Thus, the optical pickup of FIG. 1 arranged so that the light beam reflected from the optical disc 1, which is transmitted through the plate-type beam splitter 5 to the photodetector 9, can detect a focus error signal using an astigmatism induced in the light beam transmitted through the plate-type beam splitter 5.
A motor (not shown) to rotate the optical disc 1 and an optical pickup system are mounted on a deck of an optical recording and/or reproducing apparatus, such as a disc driver or a disc player, to record and/or reproduce information on and/or from the optical disc 1, using light. Since the size of the deck affects the overall size of the optical recording and/or reproducing apparatus, a particular arrangement of the optical pickup system with respect to the deck is very important, for example, in making the optical recording and/or reproducing apparatus compact.
In the case of detecting a focus error signal by an astigmatism method using the plate-type beam splitter 5 as an optical path changer instead of a separate astigmatism lens, as shown in FIG. 1, the photodetector 9 is necessarily oriented at 45 degrees to detect the focus error signal by the astigmatism method, where the optical pick system is arranged so as to have the optical path axis c parallel to a tangential direction. Here, the tangential direction refers to a direction of a series of pits or marks formed along a track on the optical disc 1.
To detect a focus error signal by an astigmatism method, as well as a tracking error signal by a three-beam method, the photodetector 9 includes a main photodetector having four or more sections, and a pair of sub-photodetectors on either side thereof. Where the photodetector 9 rotates at 45 degrees to detect a focus error signal by an astigmatism method, using astigmatism occurring at the plate-type beam splitter 5, sub-beams used to detect the tracking error signal by the three-beam method may not be received by the sub-photodetectors. Rather, the sub-beams deviate from the sub-photodetectors.
Thus, where the photodetector 9 rotates at 45 degrees, it is not possible to detect a tracking error signal by the three-beam method since the sub-beams are not received onto the sub-photodetectors. Furthermore, this eventually increases the space required to mount the photodetector 9 on the deck of an optical recording and/or reproducing apparatus since a height needed to install the photodetector 9 corresponds to a length of a diagonal line of the photodetector 9.
To solve these problems, where the optical pick-up system includes the plate-type beam splitter 5 on the deck of the optical recording and/or reproducing apparatus and is arranged to two-dimensionally arrange all components of the optical pickup system, except the objective lens 7, approximately parallel to the optical disc 1 by redirecting an optical path by 90 degrees with the reflecting mirror 2, the optical pickup system is necessarily arranged so as to have the optical path axis c inclined at 45 degrees with respect to a tangential direction. Here, the tangential direction perpendicular to the radial direction refers to a direction of a series of pits or marks formed on the optical disc 1.
However, it is difficult to arrange the optical pickup system so as to have the optical path axis c be tilted relative to the tangential direction by 45 degrees due to the size of a motor mounted on the deck to rotate the optical disc 1. In general, the optical path axis c is inclined at an angle of less than 35 degrees relative to the tangential direction to avoid contact with the motor.
For at least the reasons provided above, as shown in FIG. 4, in a conventional method, an optical pickup system is arranged so as to have an optical path axis c1 inclined with respect to a tangential direction at a predetermined angle E, for example, 35 degrees, and a photodetector 9 is placed at a location close to a rotation center Mc of a motor (not shown) mounted on a deck that rotates an optical disc 1.
Reference numeral 2 in FIG. 4 refers to a reflecting mirror that redirects a proceeding path of a light beam emitted from a light source 3 toward the optical disc 1.
Since the optical pickup system is arranged so as to have an angle of the optical path axis c1 relative to the tangential direction less than 45 degrees, for example, 35 degrees, the photodetector 9 comprises, as shown in FIG. 5, a main photodetector 9a that is split into four sections by lines d1 and d2 having oblique 5–10 degrees with respect to axes of tangential direction and radial direction, respectively, and a pair of sub-photodetectors 9b and 9c. 
In FIGS. 5, 7, and 8A–8C, an axis of the tangential direction can correspond one-to-one to a tangential direction on the optical disc 1, while an axis of the radial direction can correspond one-to-one to a radial direction on the optical disc 1.
The use of an optical pickup system arranged as shown in FIG. 4 and a photodetector constructed as shown in FIG. 5 achieves the same effect as when using a typically split photodetector for the photodetector 9 and rotating the photodetector at 45 degrees. Here, the typically split photodetector refers to a photodetector in which a main photodetector is split to be rectangular or square.
In FIG. 4, θ refers to an angle between the optical path axis c1 and the axis in the tangential direction. The optical path axis c1 forms an angle 90°-θ with the axis in the radial direction that transverses through the rotation center Mc of the motor mounted on the deck that rotates the optical disc 1.
As shown in FIG. 4, where the photodetector 9 is placed toward the rotation center Mc of the motor, the light source 3 is placed at a location farther away from the rotation center Mc than the photodetector 9 since it is not possible to position the light source 3 near the rotation center Mc due to space restrictions. The use of an optical pickup system arranged as shown in FIG. 4 and a photodetector constructed as shown in FIG. 5 allows detection of a focus error signal by an astigmatism method employing astigmatism that occurs at the plate-type beam splitter 5 used as an optical path changer, as well as detection of a tracking error signal by a three-beam method.
Thus, the conventional method of arranging the optical pickup facilitates detection of a tracking error signal by a three-beam method while making an optical recording and/or reproducing apparatus compact by reducing the space required to install the optical pickup system, as compared to a case of arranging the optical pickup system so that the optical path axis c1 is parallel to an axis of a tangential direction.
The problem with the conventional method of arranging the optical pickup system (so that the optical path axis c1 is tilted relative to the axis of the tangential direction and the photodetector 9 is placed near the rotation center Mc of the motor) arises where a single photodetector is used to detect a tracking error signal by a differential phase detection (DPD) method and a tracking error signal by a three-beam method.
CD and DVD compatible optical pickups having a single light source and a single photodetector, or having two light sources and a single photodetector, require detection of tracking error signals using both DPD and three-beam methods.
Typically, in the case of CDs, a tracking error signal detected by the three-beam method is used to perform a tracking control, whereas in DVDs, a tracking error signal detected by the DPD method is used to do the same.
However, the conventional method of arranging the optical pickup system makes it difficult to detect tracking error signals by both DPD and three-beam methods using a single photodetector based on the following aspects.
FIG. 6 illustrates that where a light beam spot is formed in a pit P (or a mark) on the optical disc 1, diffraction occurs in an edge of the pit P. Thus, as shown in FIG. 7, a light beam reflected/diffracted from the pit P on the optical disc 1 has a structure in which a 0th-order diffracted beam Lm and ±1st-order diffracted beams Ls generated by diffraction on the edge of the pit P partially overlap each other. In FIG. 7, a main light beam corresponding to the 0th-order beam and sub-beams corresponding to the 1st-order beams diffracted by the grating 4 have a partially overlapped structure since those light beams are reflected from the optical disc 1 and diffracted by the pit P. However, a reflected/diffracted light beam used to detect a tracking error signal by a DPD relates to the main light beam. Accordingly, only the reflected/diffracted light beam for the main light beam will be described hereinafter.
FIG. 7 shows a reflected/diffracted light beam (the reflected/diffracted light beam before entering the plate-type beam splitter 5) that passes through the objective lens 7 and travels toward the collimating lens 6. FIGS. 8A and 8B show the reflected/diffracted beam transmitted through the plate-type beam splitter 5. That is, FIG. 8A shows the optical path axis c1 and reflected/diffracted light beam when viewed facing the plate-type beam splitter 5, while FIG. 8B shows the optical path axis c1 and reflected/diffracted light beam when viewed facing the photodetector 9. The difference in locations of the optical path axis c1 and overlapped portions of the reflected/diffracted light beam is due to the fact that the reflected/diffracted light beam is observed in opposite directions.
When compared among the reflected/diffracted light beams in FIGS. 7, 8A, and 8B, the reflected/diffracted light beam before being transmitted through the plate-type beam splitter 5 is symmetrically transformed into that of after being transmitted through the plate-type beam splitter 5 with respect to the optical path axis c1 (or astigmatism axis). This is because the reflected/diffracted light beam transmitted through the plate-type beam splitter 5 undergoes the symmetric transformation with respect to the optical path axis c1 due to astigmatism that causes focal point variations in vertical and horizontal directions.
The reflected/diffracted light beam is symmetrically transformed with respect to the optical path axis c1 or astigmatism axis. However, for ease of understanding and illustration, the optical path axis c1 is used herein as the axis of symmetric transformation of the reflected/diffracted light beam. The astigmatism axis refers to the vertical axis in the plate-type beam splitter 5 and is inclined 45 degrees relative to the optical path axis c1 since the plate-type beam splitter 5 is tilted 45 degrees with respect to the same.
To detect a tracking error signal by the DPD method, the 0th-order beam Lm and the ±1st-order beams Ls reflected and diffracted by the optical disc 1 are evenly received by four section plates A–D of the main photodetector 9a (FIG. 5) in a normal state, i.e., where a light beam spot is formed at a center of a track on the optical disc 1.
As is evident in FIG. 8C illustrating a case in which the reflected/diffracted light beam of FIG. 8B is received by the main photodetector 9a shown in FIG. 5, a dividing line d1 of the main photodetector 9a shown in FIG. 5 is not identical to a bisector axis Ic for the reflected/diffracted light beam, and they are located in the opposite direction with respect to an axis in the tangential direction. This makes it impossible to detect a tracking error signal by the DPD method without rotation of the photodetector 9. That is, to enable detection of a tracking error signal using the DPD method, the photodetector 9 is required to be rotated at a significantly large angle.
Here, the bisector axis Ic for the reflected/diffracted light beam is defined as an axis that passes through the center of the reflected/diffracted light beam received by the main photodetector 9a and divides the ±1st-order diffracted beams Ls into two equal parts in a normal state, in which the sizes of regions where the ±1st-order diffracted beams Ls overlap the 0th-order diffracted beam Lm are identical and the overlapping regions are symmetrical with respect to the axis in the tangential direction.
In FIG. 8C, the main photodetector 9a indicated by a solid line does not rotate, while that indicated by a dotted line rotates so that the dividing line d1 is identical to the bisector axis Ic for the reflected/diffracted light beam in order to detect a tracking error signal by the DPD method.
As illustrated in FIG. 8C, in the case of applying the conventional arrangement method, the photodetector 9 needs to rotate at an angle derived by adding the absolute value of an angle θ1 at which the dividing line d1 of the main photodetector 9a is inclined relative to the axis in the tangential direction to the absolute value of an angle θ2 at which the bisector axis Ic for the reflected/diffracted light beam is inclined relative to the axis in the tangential direction.
Where the optical pickup system is arranged so that the optical path axis c1 forms an angle of 35 degrees or less with respect to the axis in the tangential direction, and the optical pickup is arranged toward the central axis Mc of the motor from the right as shown in FIG. 4 according to the conventional method, the bisector axis Ic for the reflected/diffracted light beam forms an angle of approximately 10 degrees or more with respect to the axis in the tangential direction. Since the dividing line d1 of the main photodetector 9a in the photodetector 9 is tilted at approximately 5–10 degrees with respect to the axis in the tangential direction in the opposite direction to the bisector axis Ic, the photodetector 9 has to rotate at an angle of 15 degrees or more for detection of a tracking error signal using the DPD method.
For example, where the bisector axis Ic forms an angle of approximately 10 degrees with respect to the axis in the tangential direction, since the optical path axis c1 of the optical pickup system is tilted 35 degrees with respect to the axis in the tangential direction, where the dividing line d1 of the photodetector 9 is inclined 6 degrees relative to the axis in the tangential direction, the photodetector 9 needs to rotate at approximately 16 degrees. In another example, where the dividing line d1 is inclined 10 degrees relative to the axis in the tangential direction, the photodetector 9 has to rotate by 20 degrees.
Although FIG. 8C only shows the main photodetector 9a for simplification, as the main photodetector 9a rotates, the entire photodetector 9 shown in FIG. 4 rotates.
Where the optical pickup system is arranged so that the optical path axis c1 is tilted with respect to the tangential direction and the photodetector 9 is placed toward the rotation center Mc of the motor according to the conventional method as illustrated in FIG. 4, there are many difficulties in detecting tracking error signals by DPD and three-beam methods using one photodetector 9.
That is, to detect a tracking error signal by the DPD method, the phtodetector 9 has to rotate to an extent to prevent deviation of sub-beams that detect a tracking error signal by a three-beam method from the sub-photodetectors 9b and 9c. Angles of rotation available are normally less than 15 degrees.
In the case of arranging the optical pickup system as shown in FIG. 4, since the reflected/diffracted light beam rotates in the direction in which the bisector axis Ic moves away from the dividing line d1 of the main photodetector 9a with respect to the optical path axis c1, it may be necessary to rotate the photodetector 9 at an angle of 15 degrees or more as shown in FIG. 8C so as to enable detection of a tracing error signal by the DPD method, by making the dividing line d1 of the main photodetector d1 identical to the bisector axis Ic for the reflected/diffracted light beam. Where the photodetector 9 rotates at 15 degrees or more in this manner, the sub-beams tend to deviate from the sub-photodetectors 9b and 9c, thereby making it difficult to detect a tracking error signal by the three-beam method.
That is, with the conventional arrangement of the optical pickup system, as shown in FIG. 4, it is difficult to detect tracking error signals using both DPD and three-beam methods with a single photodetector.